Chimwadzulu Hill corundum is mainly associated with soil and surface layers of weathered ultrabasic rocks originating from dunite, peridotite and amphibolite. Rich pockets of red/pink corundum have been identified in weathered bed rock. The chemistry of the intrusive rocks have produced red corundum through chromium presence while natural or blue corundum originates from containment of iron and titanium. Nickel and chrome anomalies have been identified in soils and serpentenised rocks.

Delineated or mine grid area has a total resource of 8,000 Kg of corundum averaging 75.15 g/m3 with a cut-off grade of 10g/m3. Wide surface stripping to bed rock and processing of over 100 m3 from the current 25 m3 to make the project financially and economically viable. This will mean increased mechanization and investing in high volume water abstraction or borehole drilling and building a large tailing dam.

Introduction

Chimwadzulu Hill is located 6 kilometres east of the Mozambique border about 35 kilometres from Tsangano turn off along a gravel road. The deposit is about 145 kilometres from Lilongwe. The nearest village to the hill is Katsekera.

Chimwadzulu corundum was first reported in 1958 by Bloomfield. Gunson Exports Limited did some small scale mining from 1965. In 1988 and 1989 Malawi Development Corporation (MDC) mined the deposit. Mineral Exploration PVT Limited (MINEX) took the mine in 1994 with Government assuming 15% stake in the company. The mining Licence expired in 2006 and was later acquired by Nyala Mine in 2007. Government owns 10% equity in Nyala Mines under an Agreement signed on 18th June 2008. The agreement allows Nyala mine to grant 30% of its shares to Malawians through subscription.

Geology

Chimwadzulu Hill is an ultrabasic body which intruded into quartzo-feldspathic gneiss. The ultramafic intrusive body mainly comprises of ortho-amphibolites and serpentenised peridotites that are rich in aluminum. The gneisses are Precambrian meta-sedimentary paragneisses, graphitic, calc-silicate and kyanitic in nature.

The ultramafic body seems to have altered from dunite, pyroxenite and peridotite through metamorphism, metasomatism and hydrothermal activities. These reactional activities might have introduced chromium in ruby and various ratios of iron and titanium in sapphire. Dark-reddish brown residual soils rich in corundum lie over a deeply weathered bedrock to a depth of up to 2 metres.

Reserves

Chimwadzulu hill was explored in detail between 1994 and 1999 by Mineral Exploration Private Limited (MINEX). Stuart Brand did detailed trenching, pitting and bulk sampling was done culminating into digging of 6,500 pits and 12 bulk sample pits. Power auguring was done in target areas. A reserve of 8000 kg of corundum has been delineated (Table 1).

Table 1 : Corundum Resources of Chimwadzulu Hill

DESCRIPTION

GRADE (gm/m3 )

RESERVES (Kg)

Red/Pink

6.1

250

Colourless/blue

75.2

7,750

TOTAL

8,000

Building on the work done by Stuart Brand, further work was done following discovery of discrete, rich pockets of red/pink corundum occurring in weathered bed rock. A total of about 6000 reconnaissance pits were excavated by hand on a 10m square grid to cover the area underlain by basic rocks. In some areas the grid was closed in to 5m. Red and pink corundum were quantified into indicated and inferred resources. A. C. A. Howe’s report came up with a total of 447.6 Kg of red/pink corundum summarized in the table below.

Table 2: Chimwadzulu Red/Pink corundum reserves

RESOURCE CATEGORY

DESCRIPTION

VOLUME (m3)

Grade (g/m3)

WEIGHT (Kg)

Indicated

Surface material +soil + Bedrock extensions

31,491

1.9

59.6

Inferred

Surface material +soil + Bedrock

8,862

21.2

188

TOTAL RESOURCES

40,353

6.1

447.6

Further assessment of the residual soils showed presence of chromite and nickel which could further be assessed for recovery of black sand concentrate if a mechanized processing plant was to be used. Chip sampling of weathered serpentenite rock has also recorded values in the range of 1.3 to 1.7% Nickel. This encourages exploration of bed rock Nickel and chrome resources at Chimwadzulu Hill.

The current hand digging for natural ruby is not sufficient to achieve operating profits. Worse still, less than 1% of red/pink corundum is of top grade. An attempt was made to install a heavy mineral separator to process 25m3 per operational shift. This never brought any operational profit. A plan to purchase a processing plant that can achieve 100m3 per operational shift is a progressive one and may help avoid selective mining which has not been profitable at all.

The mine will need to increase water availability, for processing, through dam construction or drilling of high yield boreholes. A large tailings dam will also be required.

The processing plant should consist of a trommel launder and jig plant with a magnet separator installed along a conveyor belt to remove magnetic minerals in order to ease extraction of gem material.

Conclusion and Recommendation

It is apparent that the Chimwadzulu Hill has potential to economically produce gem corundum, nickel and chrome if assessed in detail. An accurate survey of pit location using a GPS is necessary as all pits and trenches were located using a mine grid from a local baseline.

There is need to come up with predictive tools to identify targets for coloured corundum. The colours are associated with chromium, iron and titanium related to the original basic rocks i.e. dunite, peridotite and amphibolite. Dunite and peridotites have been serpentenised.

A resource of 8,000 kg comprised a grade of 6.1 gm/m3 for red/pink corundum and 75.2 gm/m3 for colourless/blue corundum is dominated by the latter which amounts to 7,750 Kg. The resource has not been fully quantified except where pits and trenches were dug.

Mining or excavation of soils for corundum recovery can be profitable if large volumes of soil are processed as compared to the current selective mining. Wider prospecting and deep pitting or drilling can reveal the occurrence and economic potential of gem corundum on this hill.

Credits to exploitation of corundum on Chimwadzulu Hill could be nickel and chrome found in weathered serpentines and soils that could be recovered as black sand concentrate. Platinum may also be of interest in this ultrabasic body.

References

ACA Howe International Ltd. 1999. Geology and Mineral resources of the Chimwadzulu Gem Corundum Project with particular emphasis on red and pink corundum and a review of mineral occurrences in surrounding areas, Malawi